Wearable Wireless Telemetry System for Implantable BioMEMS Sensors
- Created: Thursday, 01 May 2008
Physiological monitoring would entail minimal risk, discomfort, or restriction of mobility.
Telemetry systems of a type that have been proposed for the monitoring of physiological functions in humans would include the following subsystems:
- Surgically implanted or ingested units that would comprise combinations of microelectromechanical systems (MEMS)- based sensors [bioMEMS sensors] and passive radio-frequency (RF) readout circuits that would include miniature loop antennas.
- Compact radio transceiver units integrated into external garments for wirelessly powering and interrogating the implanted or ingested units.
The basic principles of operation of these systems are the same as those of the bioMEMS- sensor- unit/external- RF- powering- and- interrogating- unit systems described in “Printed Multi-Turn Loop Antennas for Biotelemetry” (LEW-17879-1) NASA Tech Briefs, Vol. 31, No. 6 (June 2007), page 48, and in the immediately preceding article, “Hand-Held Units for Short-Range Wireless Biotelemetry” (LEW-17483-1). The differences between what is reported here and what was reported in the cited prior articles lie in proposed design features and a proposed mode of operation.
In a specific system of the type now proposed, the sensor unit would comprise mainly a capacitive MEMS pressure sensor located in the annular region of a loop antenna (more specifically, a square spiral inductor/ antenna), all fabricated as an integral unit on a high-resistivity silicon chip. The capacitor electrodes, the spiral inductor/antenna, and the conductor lines interconnecting them would all be made of gold. The dimensions of the sensor unit have been estimated to be about 1×1×0.4 mm.
The external garment- mounted powering/ interrogating unit would include a multi-turn loop antenna and signal- processing circuits. During operation, this external unit would be positioned in proximity to the implanted or ingested unit to provide for near-field, inductive coupling between the loop antennas, which we have as the primary and secondary windings of an electrical transformer.
In the first of two parts of an operational sequence, the loop antenna in the sensor unit would receive a pulse of RF energy transmitted via the loop antenna in the external powering/ interrogating unit. This pulse would charge the capacitor in the pressure sensor and thereby excite decaying oscillations in the resonant circuit constituted by the sensor capacitance and the loop inductance. In the second part of the operational sequence, some of the power of the decaying oscillations would be coupled from the loop in the sensor unit to the loop in the interrogating unit. The frequency of the decaying oscillation would be the resonance frequency, which would vary with the sensor capacitance and, hence, with the sensed pressure. Therefore, the frequency of the signal received by the external unit during the second part of the operational sequence would be measured, and any change in the frequency from a previous value would be taken as an indication of a change in pressure.
The proposed system would offer several advantages over prior invasive physiological-monitoring sensor systems:
- The sensor materials (high-resistivity silicon and gold) would not react with body fluids.
- High-resistivity silicon would cause less attenuation of signals in comparison with other substrate materials.
- The multi-loop antenna in the external unit could be fabricated inexpensively as a printed circuit.
- The inductive-powering scheme eliminates the need for a battery in or alongside the sensor unit, thereby reducing the potential for leakage of toxic material into the patient’s body.
- Because the sensor circuit would operate only when interrogated by the external unit, power dissipation in the patient and the consequent local heating and discomfort would be minimized and the operational lifetime of the sensor unit would be extended.
- Feed-through wires for power and telemetry, used in some other systems, would be eliminated, thereby greatly enhancing the patient’s mobility and reducing the risk of infection.
This work was done by Rainee N. Simons, Félix A. Miranda, and Jeffrey D. Wilson of Glenn Research Center and Renita E. Simons of John Carroll University.
Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-18222-1.